1. Field of Invention
This invention relates to signage articles. In particular, the invention relates to low temperature formation of indicia using dry toner powder.
2. Related Art
Dry toner powder is known for printing on paper and other substrates, and compositions of dry toner powders are described in, for example, U.S. Pat. No. 5,085,918. Typically, dry toner powders comprise a colorant and a binder, and optionally a charge carrier and flow control additive. The binder is a non-tacky solid at room temperature, but melts or softens sufficiently to permanent fusion of the composition and adherence to the substrate at temperatures ranging from about 120-240.degree. C.
Various methods are known in the art for applying dry toner powder onto a substrate to form indicia. One method is to apply dry toner powder electrophotographically, wherein a charge carrier is added to the composition. In this method, a laser is used to alter the electrostatic charge on a portion of the surface of a rotating drum to form a latent image. Portions defined by the latent image accept dry toner powder from a reservoir. The rotating drum then transfers the dry toner powder in the desired image definition to a substrate contacting or nearly contacting the rotating drum. The laser can be controlled to form indicia that are generated in digital form by a computer, either directly by a computer operator, reproduced from data stored in digital form, or from an optically scanned and digitized image. Such electrophotographic imaging is commonly used in copy machines and laser printers.
After transferring dry toner powder from the rotating drum to a substrate, forming a desired image, the applied dry toner powder is fused to fix the image in place. The process of fusing involves converting the particles of dry toner powder in the transferred image to a continuous phase in which discrete particles are no longer present. Fusing aids in keeping toner within the intended edges of the image and in forming an image of acceptable quality. Dry toner powder generally is fused or fixed on a substrate by heating the dry toner powder to a temperature in the range of about 200.degree. to about 220.degree. C. for approximately 0.1 seconds, although temperatures approaching 220.degree. C. typically are required to obtain satisfactory image quality.
For example, the '918 patent describes articles wherein the substrate onto which the dry toner powder is transferred in the form of a desired image is retroreflective signage, wherein the dry toner powder is fused to form the image through a high temperature process. Optionally, there is disclosed a clear polymeric film formed over the image bearing surface of the signage articles, e.g., by dip coating or laminating, to provide a protective coating.
HP Laser Jet 4 and 3M Printers User's Manual (First Edition: October 1992) and HP Laser Jet IIP Printer User's Manual (First Edition: June 1989) indicate that substrates, as well as pigments and other components of which the substrate is comprised, must be able to tolerate fusing temperatures in excess of 200.degree. C. for at least 0.1 seconds without physical or chemical deterioration. Similarly, Siemens Nixdorf Electronic Printing Systems Manual (January 1992 Edition) states that pigments and substrates must be able to tolerate temperatures of at least 200.degree. C. at a pressure of 2.4.times.10.sup.5 Pascals (Pa). Fusing at such temperatures and pressures limits the composition of substrates to which dry toner powder can be applied since a substrate must tolerate the temperatures used for fusing without noticeable chemical or physical degradation.
In attempting to develop additional processes for fusing of dry toner to a substrate, high pressure, in the range of about 14 megaPascals (MPa) to about 28 MPa (2000 to 4000 psi), has been used to achieve fusion at lower temperatures. Toward that end, an ideal dry toner fusion process would require no machine warm-up time, a minimum power input and reduced fuser roll degradation. Unfortunately, the fusing pressures, such as those indicated above, required for adequate print quality cause considerable compressive damage to the substrate. This type of damage results, for example, in the calendering of paper, and an unsatisfactory glossy appearance.
Numerous attempts have been made to utilize a noncontact fusing technology where no surface touches the dry toner powder side of the paper until fusing is complete. For example, unfused dry toner powder may be exposed to solvent vapors which soften the binder in the dry toner powder and thus accomplish fusing, for example, as disclosed in U.S. Pat. No. 2,684,301 (Mayo). U.S. Pat. No. 4,311,723 (Mugraner) also discloses a vapor fusing system that utilizes a trichlorotrifluoroethane and either acetone or methylene chloride in an azeotropic mixture.
Fusing dry toner powder at high temperatures or at high pressures also imposes the additional requirement of complex equipment that is capable of exerting uniform heat or pressure upon the substrate surface. On the other hand, fusing dry toner powder to a substrate by solvent vapors is limited to substrates that would not be damaged by the chemical action of the solvents, and necessitates the additional processing step of passing the substrate through a chamber containing the solvent vapors. Furthermore, the use of solvent vapors is likely to have undesirable environmental consequences, and printing mechanisms using such chemically-based using processes may, in turn, be subject to significant regulation relating to release of the solvents into the atmosphere.
Thus, there exists a need to provide a low-temperature, low-pressure, solvent-free chemical fusion process that is relatively simple and economical to implement.